answer; the tunica intima also called tunica interna, consists of elastic fibers,connective tissue and smooth muscle cells;
The best answer would be:
In Pavlov's experiment on conditioning the food was the unconditioned stimulus that triggers a reflex or natural behavior. Think of it like something that makes you do something automatically.
For example, the food. When the dog smells food, it salivates. This is unconditioned because without teaching the dog, it automatically responds by salivating.